Lesson 5: Life's History: The Great Adventure 5.1 Classification, Natural Selection, and the Quanta of Inheritance Figure 5.1.1 Gregor Mendel The first geneticist. Mendel demonstrated that physical characteristics are passed from parent to offspring in an orderly and predicatble manner. The notion that the world was created as it is and has since existed more or less unchanged is one that resonates deeply with the thought that the world was made for us, and that we are in the center of it. It is a world view that goes back to the Stone Age. In the last 200 years or so it has become entirely unacceptable to science. We now think that Earth is a rotating sphere orbiting around a rather unremarkable star, one like billions of others in the galaxy, which in turn is one among billions of galaxies. We also think that Life on Earth has a history reaching back billions of years. It is a world view that includes geologic history and fundamental change through time. In reconstructing the history of life there are a number of quite different approaches. One is based on the fossil record, that is, the altered remains of organisms in sedimentary rocks. Such fossils range from mineralized bones and shells to chemical compounds both inorganic and organic. They also include tracks, burrows and feces. They include certain types of sedimentary rocks, such as "banded iron formations" and limestones and chert layers. Entire mountain chains consist of the remains of microscopic organisms, as was realized in the 19th century. Other approaches are based on classification of living organisms, based on morphology or chemistry or both. The chemical approach is especially powerful because it analyzes the genetic makeup of an organism. Organisms that have similar makeup have a common ancestor, and share a common history from the origins of life up to the common ancestor. Well before the geologic record was interpreted in terms of evolution, the task of classifying animals and plants demanded detailed observation of structure and of the life cycle of individual organisms. Aristotle (384-322 B.C.) classified more than five hundred animal species and arranged them into hierarchies. He dissected some fifty of them. He noted that dolphins bear live young and suckle them like the terrestrial mammals. Thus, he classified them with the beasts of the field rather than with the fish of the sea. The meaning of Aristotle's observation that dolphins are descended from land mammals that returned to the sea and changed their shape and habits to live among fish only became clear some two thousand years later. Much resistance had to be overcome before evolution could be accepted as a fact. We still marvel at the powers of observation of the Swedish naturalist Carl von Linneaus, (1707-1778), who introduced the modern nomenclature and a systematic taxonomy. He was brilliant in recognizing different species in plants and animals, and he grouped them into "genera" and "families" according to similarity. Yet, he was unable to accept the obvious idea that similarity implies common ancestry. It was the French naturalist Jean-Baptiste de Lamarck (1744-1829), two generations later, who insisted on evolution as the reason for the diversity of animals (in 1809). His insights came from studying and classifying animals other than vertebrates (then referred to as "insects and worms", now called "invertebrates".) Lamarck, suggested that evolution proceeded through adaptation of organisms to their environment (rather than by some drive toward a goal, as proposed earlier by the philosopher Immanuel Kant). It has become popular to dismiss Lamarck by depicting his proposed mechanism of evolution as a dead end, in the form of "inheritance of aquired characteristics". It is not entirely clear, actually, what he meant to convey in terms of our current categories of thinking about inheritance. The concept of "inheritance of useful characteristics" which is equally implied in Lamarck's writings, is surely acceptable. In fact, it provides the basis for the concept of "natural selection", introduced by Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913), the mechanism for driving evolution that has become closely identified with evolution itself. Why was Lamarck ignored for 50 years, while Darwin hit the jackpot with his book? Being in the right place at the right time helps. Also, Lamarck had the famous Baron Cuvier for a colleague (who rejected evolution and totally ignored him). Darwin had the eminent zoologist Thomas Henry Huxley for a friend (who vigorously promoted evolution). Messing with Huxley was to invite trouble. Thomas Henry Huxley's answer to Bishop Wilberforce, in a debate on Darwinian evolution: Upon being asked, in public debate, by the mathematician and Anglican Bishop Samuel Wilberforce, whether he (Huxley) claims descent from monkeys through his paternal or maternal grandparents, Huxley answered as follows: "If, then, the question is put to me, would I rather have a miserable ape for a grandfather, or a man highly endowed by nature and possessing great means and influence, and yet who employs those faculties and that influence for the mere purpose of introducing ridicule into a grave scientific discussion, I unhesitatingly affirm my preference for the ape." Darwin's book (The Origin of Species by Means of Natural Selection or the Preservation of Favored Races in the Struggle for Life, 1859) is generally reckoned as the start of modern biology. It has the great advantage, compared with many earlier writings on biology, that it makes pragmatic and sensible inferences from mountains of observations. It has the even greater advantage of putting forward a simple and easily understood mechanism to drive evolution, that is, Natural Selection. It has the disadvantage that it puts forward a culling mechanism as being in charge of evolution, without offering a mechanism to generate new variety to cull from; our present understanding of genetic mutation was still far in the future. While natural selection was eventually accepted as the process governing evolution, it says nothing about how variation arises in the first place, and how a characteristic appearing in a single individual can be preserved. Why is not such a property diminished by cross-breeding with other individuals not having the property? The Austrian botanist (and Augustinian monk) Gregor Johann Mendel (1822-1884) carefully observed the patterns of reproduction of peas in the monastery garden. He found that inheritable traits (shape, color) are passed from one generation to the next in indivisible, unmixable quanta. Work on fruit flies, by the American biologist Thomas Hunt Morgan (1866-1945) showed that these units of inheritance are present within the chromosomes of the cell nucleus. The Danish botanist Wilhelm Ludvig Johannsen (1857-1927) called these units "genes", and the term was generally adopted. Many types of genes are shared by organisms as different as frogs and flies, and even across kingdoms. The similarity of genetic material has become the new standard in measuring relatedness between organisms.